In general, a programmable logic controller (PLC) includes a display device using a light emitting element as a display unit for displaying a state of operation or the like of a device. The light emitting element used for the display device is generally a light emitting diode (LED). Namely, in general, the PLC includes an LED display unit. The LED display unit includes a plurality of LED chips which are mounted on a printed board (generally, referred to as an LED board) and arranged in a matrix shape and is covered with an LED cover formed with a semi-transparent resin material. Characters are printed on a front surface of the LED cover so as to correspond to the LED chips arranged in a matrix shape on the LED board, and portions adjacent to the characters are enabled to emit light by allowing light (LED light) emitted from the LED chips to transmit from a back surface to the front surface of the LED cover, so that the state of operation or the like can be displayed in the LED display unit.
In recent years, miniaturization and weight reduction of a programmable logic controller are required, and the area of an LED display unit is also reduced. Particularly, reduction in width-direction dimensions of the programmable logic controller is greatly required. Therefore, the mounting interval of LED chips on an LED board is also reduced.
If the mounting interval of the LED chips is reduced, there is a problem in that visibility is easily deteriorated, for example, overlap of characters printed on the LED cover and the LED light occurs, or misrecognition in adjacent LEDs occurs when the LED display is viewed from a slanted side.
As an invention relating to improvement of visibility of the LED light in the LED display unit, there are inventions disclosed in Patent Literatures 1 and 2.
In the invention disclosed in Patent Literature 1, a cylindrical member is installed between an LED and a display panel so that the position of light is not shifted when the display panel is viewed from any position in front of the display panel.
In the invention disclosed in Patent Literature 2, a grid-shaped LED filter is disposed at a surface side of an LED board to partition a light emitting area for each LED, so that visibility is intended to be improved.